1. Field of the Invention
The present invention relates to an improvement in the process of making defluorinated phosphate rock granules.
2. Description of the Prior Art
Phosphate-containing materials are in great demand for use both as plant fertilizers and as animal feed supplements, and the market for such materials is constantly growing. The principal sources of phosphate in the United States are the great natural deposits of pebble rock and phosphate rock found in Florida and in the western states, and such widely distributed phosphatic materials as apatite. However, these naturally occurring phosphate materials contain combined fluorine in quantities which can, under certain conditions, interfere with the availability of the phosphate values when used as fertilizers and which are detrimental to health when used as animal feed supplements. As a result, when these phosphatic materials are to be used as fertilizers, and particularly when they are to be used as animal feed supplements, the phosphatic material is commonly treated to reduce the fluorine content of the material usually below a specified minimum amount. For instance, the fluorine content of an acceptable animal feed supplement today should be less than one part of fluorine per 70 parts of phosphorus by weight.
A great deal of effort has been devoted to the problem of developing economical and efficient processes for reducing the fluorine content of these phosphatic materials (herein collectively referred to as phosphate rock) to acceptable limits. Thermal processes have been the most widely known and commercially successful means of defluorinating phosphate rock. A typical thermal process involves calcining the phosphate rock in the presence of phosphoric acid, sodium carbonate, water vapor, and other reagents at a temperature in excess of about 1000.degree. C. to drive off a substantial portion of the fluorine content of the rock and thereby obtain a defluorinated phosphate product. The principal difficulty encountered in the defluorination of phosphate rock by high temperature calcination arises from the fact that the temperature required for the removal of fluorine is so high that objectionable fusion or sintering of the rock can take place before removal of fluorine to the desired extent is effected. In order to avoid the disruption of the calcination process due to partial fusion and the consequent formation of large lumps of phosphate-containing material in the kiln or other calcining apparatus, it has heretofore been the practice to form the defluorination feed material into nodules, agglomerates, or granules having sufficient mechanical strength, porosity, and resistance to fusion to withstand calcination in a rotary kiln, and a great deal of time and effort have been expended in developing methods for making such a granulated feed material. The prior art practice of making defluorination feed material is exemplified by the processes described in U.S. Pat. No. 2,995,436 and U.S. Pat. No. 3,189,433 both issued to Hollingsworth et al, and co-pending U.S. patent application Ser. No. 761,049 filed Jan. 21, 1977 to Harold V. Larson.
The process disclosed in this latter reference represents a particularly useful technique for making defluorinated phosphate rock granules, especially from relatively low grade phosphate rock concentrates. That disclosed process is similar to the one shown by the FIGURE of the present application. As can be seen from the FIGURE, one feature of both that process and the present process is to recycle a portion of the dried granules (before calcining) back to the granulation zone to establish therein a bed of dried recycled solids which are sprayed with a wet feed slurry contained fine particles of fluorine-containing phosphate rock therein. The dry solids are thereby coated with relatively thin successive coatings of rock slurry, thus forming wet granules having a substantial portion of fluorine-containing phosphate rock therein. Each time a dry particle is recycled back to the granulation zone, the size of the dry recycled solids, thus, gradually increases. The principal mechanism by which this size-increasing is accomplished is called "layering" whereby the dry recycled solids act as nuclei for the smaller particles of phosphate rock contained in the wet feed slurry. In particular, for each pass through the granulation zone, one or more layers of these smaller phosphate rock particles form a layer around each dry nuclei. These new layers are held in place by the liquid present from the feed slurry and when the liquid is removed in the subsequent drying step, a new larger granule is formed. A more detailed discussion of this "layering" mechanism is found in an article by P. V. Sherrington entitled, "Liquid Phase Relationships in Fertilizer Granulation By A Layering Process" in the Canadian Journal of Chemical Engineering, Volume 47, pages 308-316, June, 1969.
Conventional granulation techniques for making defluorinated phosphate rock products exemplified by the processes disclosed in the above references are widely used by the fertilizer and animal feed industries today and relatively large commercial levels of granulated products are made. Under normal conditions, these processes operate quite smoothly and relatively good yields of product are obtained. However, occasionally these are upset by problems that are either beyond the control of the operators or caused by human error. One particular troublesome is the presence of an abnormally large percentage of very small-sized (for example, under -20 mesh) solid particles in the dryer discharge. As shown in the FIGURE, such fine-size particles are normally recycled back to the granulation zone, and therefore the excess fines cause lower yields in product because they are always recycled. Alternatively, if the excess fines are allowed to be fed into the calciner, more problems may be created. For instance, if fines are charged to the calciner, short circuiting across the fluidized bed or increased carry-over in the flue gas may also result. The short circuiting may cause decreased reaction time of the granules in the calciner with consequently higher fluorine in the product. Increased carry-over may result in more material passing by the cyclone and entering the scrubbers. This in turn may cause solids accumlations in the scrubber recycle liquor and excessive pump wear. In addition, high fines in the calciner feed may accelerate fouling of the calciner and thereby decrease unit on-stream time.
Furthermore, as will be discussed in detail below, while these problems of increased recycled fines and fines in the calciner feed are quite serious, what is even more troublesome about this excessive fines problem from the dryer is that once it starts, an increasingly large percentage of fines will discharge from the dryer unless a change in the process is made. If not corrected, eventually no granules will be produced and the whole system will be clogged with fines. This results in a major shutdown and a large loss in product. Therefore, a need exists in this art for means to correct this problem quickly when it occurs and return the process back to its normal rate of production. The present invention is such a solution.